Sliding vane rotary pumps are well known for use in a variety of applications, and are particularly common in aeronautical and aviation applications. These devices are commonly used, for example, in powering gyroscopically controlled flight instruments in airplanes. Proper maintenance to prevent the failure of these devices is therefore very important.
Sliding vane rotary pumps typically comprise a rotor, a housing or stator, and a plurality of vanes. The rotor includes radial slots which receive the vanes, and can be constructed of either carbon, carbon composite, or hardened metal materials. The stator is typically constructed of a hardened metal material and the interior comprises an ellipsoidal cavity which receives the rotor. The vanes are typically constructed of carbon or carbon composites, and, as the rotor rotates, are caused by gravity to slide in and out of the slots as the rotor moves within the cavity, extending and retracting synchronously with the relative rotation of the rotor to provide compression and expansion of the air and therefore to produce a pumping pressure.
The sliding motion of the vanes, however, results in a significant degree of friction which is exacerbated by the atmospheric pressures induced when used in aviation applications. While a number of efforts have been made to mitigate the frictional wear from the sliding vanes, including, for example, employing various coatings on both the vanes and the rotor, wear and breakage of the vanes remains problematic, as the friction developed between the vane and the rotor is eventually destructive to the pump. As the vanes reciprocate in the rotor slots, friction causes the vanes to wear, eventually shortening the vanes until they no longer reciprocate in the slots properly. Eventually, the shortened vanes lead to pump failure. Although the wear on the vanes can be monitored to some extent on the aircraft's vacuum gauge, which provides an indication if the pump is not operating correctly, there is generally little warning of a pending failure, as pump performance and efficiency are generally unaffected by wear on the vanes until a total or near total failure occurs.
To prevent such failures, the operation time of the pump is monitored and the number of hours of operation is used as a benchmark for determining when to replace the pump. While generally effective in preventing pump failure, replacing the pump based entirely on hours of operation is expensive, resulting in premature pump replacement even when no significant wear has occurred, and further incurring costs in the form of maintenance time, down time for the vehicle and equipment expense. To reduce the equipment costs, the pump can be removed from the equipment and disassembled to evaluate the amount of wear. Again, however, this operation requires a high degree of maintenance activity, significant vehicle down time, and is highly dependent on the opinion of the evaluator.
Another alternative, in which the length of the vanes are visually monitored, has been proposed in U.S. Pat. No. 6,450,789. Here, a “view port” is provided in a back of the housing of the pump. Through the view port, a maintenance person can examine the length of the vanes with reference to the width of the port and/or an associated calibration hole. While providing a means for viewing the rear on the vane, the view port can be difficult to access in the vehicle, and further requires a judgment call on the part of the maintenance personnel examining the pump.
There remains a need, therefore, for an easy, consistent and effective way to gauge the wear on a vane in a dry air sliding vane pump without requiring removal or disassembly of the pump.